PBM-1607: Improve logging when reporting deletion error

[boris-ilijic]

Currently, when pbm-agent fails to delete an unsuccessful backup, it swallows the error and reports just:

E [backup/2025-10-21T17:30:34Z] Failed to delete leftover files for canceled backup "2025-10-21T17:30:34Z"

After this change, the actual error will also be logged.

[Copilot]

Pull Request Overview

This PR improves error logging in the PBM (Percona Backup for MongoDB) agent by ensuring that actual error details are included when backup deletion fails, rather than being swallowed. The change addresses the issue where only a generic message was logged without the underlying error information.

Key changes:

• Enhanced error message to include the file name when fetching file parts fails
• Modified deletion error logging to include the actual error details

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pbm/storage/split_merge_mw.go	Added file name to error message when fetching file parts base fails
pbm/backup/backup.go	Updated error logging to include actual error details when deletion fails

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